Carburizing is a heat treatment process in which iron or steel absorbs carbon liberated when the metal is heated in the presence of a carbon bearing material with the intent of making the metal harder. Depending on the carburizing time and temperature, an affected area can vary in carbon content. Longer carburizing times and higher temperatures lead to greater carbon diffusion into the metal as well as an increased depth of carbon diffusion. When the iron or steel is cooled rapidly by quenching, the higher carbon content on the outer surface becomes hard via the transformation from austenite to martensite while the core remains soft and tough as a ferritic and/or pearlitic microstructure. Carburizing is most commonly used on low-carbon workpieces which are placed in contact with a high-carbon gas, liquid or solid. It produces a hard workpiece surface with a case hardness depth of up to 10 mm and a tough and ductile workpiece core.
The volume change that occurs between the carburized area (case) and the base material (core) of a metal creates compressive residual stress (CRS). It can be desirable to create maximal compressive stress in a metal. Over-carburizing a metal may however result in a risk of quench cracking, high surface retained austenite, dimensional instability due to martensite contraction, and low CRS.